1. Field of the Invention
This invention relates to a steam-sterilizable flexible pouch package, to a method of using the pouch, and to apparatus which facilitates use of the pouch. More particularly, the invention relates to such pouches which are steam-sterilizable and preferably sterilized in vacuum cycle sterilizers with steam at high temperatures, typically at least about 270.degree. F. and which are reliably openable, by peeling, after such sterilization. Still more particularly, the invention relates to such packages which are sufficiently large and robust to permit such sterilization of heavy objects, such as a tray bearing medical devices or instruments.
2. Description of Related Art
Various medical instruments and supplies are subjected, prior to use in hospitals, to sterilization treatments such as, for example, sterilization with a sterilizing fluid such as steam, ethylene oxide gas, or hydrogen peroxide plasma, radiation sterilization, and dry-heating sterilization. Ordinarily, the first step in sterilization is to package or wrap the unsterile device before exposing it to a sterilant. Generally, packages are used for sterilization of small, lightweight objects and wrapping is used for sterilization of heavy objects, particularly metal trays in which one or more objects are held. Such trays are usually made of stainless steel and typically weigh from 5 to 16 pounds, and often up to 30 pounds, when loaded.
In all sterilization treatments, there is a general requirement to observe a sterile technique. In the context of packaging or wrapping, it is particularly important that sterile technique be practiced during unwrapping or during opening of a package. For example, sterile technique does not allow a hand or object to contact a sterile item. It is important, therefore, that any flexible wrapping or packaging material have good drapeability such that it will readily fall away from a sterilized item as the item is unwrapped or as a sterilized package is opened.
A current practice of wrapping a tray for steam sterilization employs a double-layered muslin cloth, commonly known as CSR wrap, that is secured around an unsterile medical device by taping. The wrap permits the steam to penetrate into and out of the interior of the cloth wrap, but acts as a barrier to bacteria and other organisms after sterilization. Normally, a dust cover is also employed in the post-sterilization phase. CSR cloth wrap, however, is prone to fluid strike-through and exhibits tearing with extended use. In addition, steam sterilization methods employing cloth wrap are normally practiced by first unwrapping the sterilized tray, followed by moving the unwrapped tray to an area where the tray contents will be used. Sterile technique requires careful and precise procedures. Accordingly, cloth wrap practice is time consuming and expensive.
An alternative practice replaces muslin wrap with a disposable non-woven CSR wrap that is applied in the same manner, i.e., double-layering of wrap secured by taping and with the use of a dust cover. More recently, a single-layer disposable non-woven CSR wrap has been proposed. Although non-woven CSR wraps offer both improved fluid resistance and improved bacterial barrier migration over their cloth counterparts, sterilization practices that use these non-woven materials are still labor intensive and costly.
Steel or plastic self-contained reusable trays fitted with an outer housing that has a replaceable filter have also been employed as sterilization containers. While the steel tray system offers excellent sterility maintenance, it is not an attractive practice for many hospitals because the containers are extremely expensive. Plastic trays, unlike the steel trays, allow for visual inspection and identification of contents by employing a transparent plastic. Although to a lesser degree than the steel counterpart, plastic self-contained sterilization trays are also expensive. Moreover, sterilization trays have considerable mass which gives rise to a problem of sterilant condensate which arises with this method of sterilization.
Sterilization pouches and bags comprised of paper and plastic webs have found wide applications in hospitals. Plastic webs may afford easy identification of contents through a transparent plastic web. However, as mentioned above, these applications have been limited to sterilization of small or lightweight objects that are easily packaged.
Kraft paper, both coated and uncoated, is used in such packages for both steam and gas sterilization but is not well suited for heavy objects. Moreover, opening a sealed paper package gives rise to the generation of loose paper fibers, which is undesirable in an operating room or other area where dust is desirably kept to a minimum.
Non-woven materials have been substituted for paper in such packages. For example, sheets of spun-bonded polyolefin such as "Tyvek" are widely used in packages for gas sterilization. "Tyvek" offers greater drapeability and reduced fluid strike-through as compared to Kraft paper. However, Tyvek will not withstand high temperature steam autoclaving, and is not employed as an alternative to CSR wrap in high temperature steam sterilization.
It is obviously important that a sterilizable package be reliably sealed, and that it remain sealed after sterilization such that its sterilized contents remain sterilized for the required time, generally for at least 30 days in the case of hospital instruments. It is also important, however, that the sealed sterilized package is reliably opened without requiring excessive force and without the risk of generating fiber "dust". Peelable heat seals or "peel seals" between opposed plastic webs have been proposed as being suitable for both of these important properties. For example, Sellers, U.S. Pat. No. 3,410,395, discloses a package which comprises a laminar sheet material folded onto itself or assembled with another separately formed sheet or panel and heat sealed to form a pouch. The laminar sheet material comprises a perforated heat sealable film, preferably polyethylene, which is laminated with and bonded to a paper sheet. Steam can penetrate both the paper layer and the perforated film layer to enter the pouch. The heat seal between the facing panels or sheets is said to be peelable and the package is said to be able to withstand steam sterilization and able to be peeled open by the application of a moderate opposing pulling force.
At the present time, most steam sterilization of medical instruments and the like is carried out in hospitals and medical facilities with saturated steam. Sterilization, effected in a vacuum cycle autoclave, has three phases: preconditioning; exposure; and drying. In the preconditioning phase, after placing the object(s) to be sterilized in an autoclave, saturated steam can be introduced at elevated pressure, typically at least 10-15 psig, followed by evacuating the autoclave to an absolute pressure of about 10-20 inches of mercury. The autoclave is also heated by a steam jacket, with jacket steam typically at a temperature of about 250-275.degree. F. These steam/vacuum conditioning pulses are usually repeated at least once and often three or four times, and the total preconditioning time is at least 4-5 minutes and up to 30 minutes or longer. In the next, exposure, phase saturated steam is introduced and pressure raised and to mention saturated steam at a target temperature and for a predetermined period of time. In gravity steam sterilization, a typical target temperature of about 250.degree. F. is typically maintained for up to 20 minutes or more. Most steam sterilization of packaged or wrapped medical instruments is carried out in hospitals and medical facilities in vacuum cycles with saturated steam at much higher target temperatures, usually at least about 270.degree. F., and with an exposure time of usually at least three minutes. In the subsequent drying phase, the pressure is reduced and autoclave is again evacuated, typically to an absolute pressure of about 10-20 inches of mercury or less, with continued heating via the heated jacket. Jacket temperature is typically 250-275.degree. F. The drying phase typically lasts for at least about 5-10 minutes and may be as long as sixty minutes or more. Autoclave equipment for effecting such steam sterilization is widely available commercially and includes complete automation for carrying out the process under predetermined conditions. The present invention has particular applicability to flexible film packages which are suitable for autoclave sterilization at high steam exposure temperatures of at least 270.degree. F. These conditions are referred to hereinafter as high temperature vacuum cycle steam sterilization conditions. The packages disclosed by Sellers are incapable of undergoing sterilization at these conditions.
Wilkes, U.S. Pat. No. 4,367,816 discloses a package which is said to be gas sterilizable, and no mention is made of steam sterilization. This package comprises a low density polyethylene sheet which is heat sealed to a laminar tear strip made up of a gas-permeable paper sheet bonded to a perforated plastic film, the latter film being a sheet of high density polyethylene coated on both sides with a thin layer of a blend of ethylene vinyl acetate and low density polyethylene. The latter layer is heat sealed to the low density polyethylene sheet to form a peelable heat seal. Again, however, the materials will fail if the package is subjected to conventional high temperature steam sterilization conditions. A chevron type package is illustrated in FIG. 7.
Various types of line heat seals which join heat-sealable plastic webs are known in the art. Several of these line heat seals are referred to as "peelable" seals or "peel" seals. A peelable line heat seal is a line heat seal which is openable along the line heat seal by pulling apart heat sealed webs. At least three distinct types of peelable line heat seals are known. In a "true" peel seal, the heat sealed webs separate at their heat sealed interface, with little or no transfer of material from one web to the other. In a "cohesive failure" peel seal, as the heat sealed webs are peeled apart, there is a cohesive failure of only one of the heat sealed webs. The failing heat sealed web tears in a plane which is generally parallel with the plane of the heat seal, Some of the cohesively failed heat sealed web transfers to the web to which it is heat sealed, and some remains behind with the failing heat sealed web. In this type of seal, the strength of the failing heat sealed web is weaker than the heat seal itself. A "delamination failure" or "adhesive failure" peelable heat seal is similar to a cohesive failure seal in that a portion of a failing heat sealed web transfers to the non-failing web. In a delamination failure peel seal, however, the failed web is a lamination, and failure occurs in a predictable location, namely between laminae. In the simplest case, the failing web is a two layer lamination, one layer of which is a heat seal layer which is permanently heat sealed along a permanent line heat seal to the other web. When the heat sealed webs are pulled apart, the failing web readily delaminates in the region of the permanent line heat seal because the permanent line heat seal which bonds the two webs is stronger than the bonding strength between the layers of the failing web. Pulling the webs apart also causes the failing layer to tear at locations just outside and just inside the line heat seal because the bonding strength between the layers of the failing web is stronger than the tear strength of the heat seal layer of the failing web. Thus, the portion of the heat seal layer of the failing web, in the region of the line heat seal, clearly separates by delamination and tearing in a predictable fashion from the layer to which it was laminated, and this separated portion of the failing heat seal layer is transferred to the non-failing web. Thus, failure is generally in a plane which is parallel to the plane of the permanent line heat seal, and there is a transfer of the failed heat sealed layer, in the region of the permanent line heat seal, to the web which did not fail. However, in a delamination failure peel seal, failure proceeds predictably at the interface of the failing lamination and in the region of the permanent line seal.
The present invention relates to sterilizable pouch packages utilizing a delamination failure type of peelable line heat seal.